1. Field of the Invention
The present invention relates to a pelletizer preferably in the form of an underwater pelletizer. In particular, the invention relates to a cutter and/or grinding head for such pelletizer, including a tool carrier which can be rotatably driven about a tool carrier axis of rotation, and at least one cutting and/or grinding tool, which is attached to the tool carrier and is spaced from the tool carrier axis of rotation, for knocking off plastic melt emerging from a pelletizer die plate and/or for grinding the pelletizer die plate.
2. Description of the Related Art
Pelletizers, such as underwater pelletizers, usually have a pelletizer die plate with a plurality of bores or channels, through which plastic melt is pressed in the form of a strand. On the outlet side, the emerging strands of plastic melt are knocked or cut off by a rotating cutter head, so that plastic pellets are obtained, which in the case of an underwater pelletizer are carried away by the process fluid flowing around the cutting head.
Some of the factors that affect the quality of the plastic pellets and of the pelletizing process include the shape and surface accuracy of the die plate on its outlet side, and the precise geometrical interaction between the outlet side of the die plate and the cutting or knock-off tools passing over the same. If wear results in irregularities on the die plate outlet side, the plastic material can start to smear on the die plate outlet side, impairing the neat cutting or knocking off of the pellets. One known approach to avoid or correct this problem has been to surface-grind the die plate of the pelletizer from time to time and, instead of the cutter head, use a grinding head with an end-face abrasive coat which is urged against the outlet side of the die plate in order to surface-grind the latter. However, the surface accuracy and fineness that can be achieved is limited and, due to the usual uniaxial rotatory grinding movement, annular ridges can be cut into the die plate surface.
In addition to wear on the die plate, wear on the cutting tools also can impair the pelletizing process. Usually, cutting plates which can be mounted in cutting plate holders at the desired angle are used as cutting tools so that they can pass over the die plate in the specified angular position, in order to knock off the emerging melt. In this regard, it is known to use turnover plates, i.e. cutting plates that can be turned over in the cutting plate holders, so that both sides of the usually rectangular cutting plates can be used, until they must be discarded due to excessive wear. The stability of the cutting plates is, however, limited and, when changing the cutting surface, differences in shape with respect to the ground outlet side of the die plate sometimes are obtained.